The present invention relates to a confidential document reproduction prevention method.
Recently, in many fields, it is demanded that a huge quantity of information be quickly exchanged between a number of people and transmitted to them. For that purpose, copying machines for copying documents, such as electrophotographic copying apparatus, and information transmission and recording apparatus, such as facsimile apparatus, are developed and widely used. At present, there are substantially no difficulties in making copies from ordinary documents. Further, in accordance with the remarkable development of such copying machines and information transmission and recording apparatus, the quantity of information transmitted is increasing rapidly. On the other hand, the remarkable development of copying machines and facsimile apparatus has brought about a new problem that confidential documents, for instance, new product planning documents, market research documents, diplomatic confidential materials or military confidential materials, are secretly copied without difficulty and those copies are stolen.
Under such circumstances, several methods of preventing reproduction of confidential documents by a conventional electrophotographic copying machine have been proposed.
In one method, confidential documents are prepared by writing or printing confidential image information with a fluorescent material. In this method, even if it is tried to make copies from the documents by a conventional electrophotographic copying machine, when the documents are exposed to light during the exposure process, the image areas emit light because of the presence of the fluorescent material in the image areas, while the non-image areas normally reflect light therefrom. As a result, the image areas and non-image areas cannot be optically distinguished from each other, and normal latent electrostatic images corresponding to the confidential original images cannot be formed on the surface of a photoconductor of the electrophotographic copying machine. The result is similar to that of the case where a copy is made from a blank sheet which bears no images.
In another method which utilizes substantially the same principle as in the above-described method, the entire original-image-bearing surface of the confidential document is coated with a fluorescent material. When the document is exposed to light for formation of the latent electrostatic images corresponding to the original images, light is emitted from the entire surface of the document coated with a fluorescent material. As a result, the original images become substantially invisible during the exposure process.
In a further method, confidential documents which bear confidential image information thereon are made of thin metallic sheets with an extremely smooth surface. Since light is reflected almost uniformly from the entire smooth surface during the exposure process although the smooth surface bears image information, it is impossible to form the latent electrostatic images corresponding to the original images from the shiny original.
In these conventional methods, there is employed a technique of minimizing the difference in density between the image area and non-image area or of making the difference substantially zero when exposed during the image exposure process, thus making the confidential document look as if it is a blank original which bears no images thereon at the time of exposure. A significant shortcoming of these confidential documents is that they are difficult to read.
Conventional photoconductors for use in electrophotographic copying machines are designed so as to have photosensitivities which are as close as possible to tne photosensitivity of the human eye, in order to make faithful copies from originals which bear images visible to the human eye. Under such circumstances, confidential documents which hardly allow legible copying by the conventional electrophotographic copying machine are suitable as confidential documents in order to prevent the copying thereof, but they are usually difficult to read and not suitable for normal reading, whereas documents which allow legible copying are easy to read, but not suitable as confidential documents which prevent the copying thereof.